1. Field of the Invention
The present invention relates to a field emission device for emitting electrons from an emissive material and, more particularly, to a field emission device having an improved electron emission performance, which can be used for high-resolution field emission display.
2. Discussion of the Related Art
Field emission displays (FEDs) are new, rapidly developing flat panel display technologies. Compared to conventional technologies, e.g., cathode-ray tube (CRT) and liquid crystal display (LCD) technologies, FEDs are superior in having a wider viewing angle, low energy consumption, a smaller size, and a higher quality display. In particular, carbon nanotube-based FEDs (CNTFEDs) have attracted much attention in recent years.
Carbon nanotube-based FEDs employ carbon nanotubes (CNTs) as electron emitters. Carbon nanotubes are very small tube-shaped structures essentially composed of a graphite material. Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes can have an extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (potentially greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). Thus, carbon nanotubes can transmit an extremely high electrical current and have a very low turn-on electric field (approximately 2 volts/micron) for emitting electrons. In summary, carbon nanotubes are one of the most favorable candidates for electrons emitters in electron emission devices and can play an important role in field emission display applications.
Generally, FEDs can be roughly classified into diode type structures and triode type structures. Diode type structures have only two electrodes, a cathode electrode and an anode electrode. Diode type structures can be used in characters display, but are unsatisfactory for applications requiring high-resolution displays, such as picture and graph display, because of their relatively non-uniform electron emissions and difficulty in controlling their electron emission. Triode type structures were developed from diode type structures by adding a gate electrode for controlling electron emission. Triode type structures can emit electrons at relatively lower voltages.
FIG. 1 is a schematic view illustrating a conventional triode type field emission device 4, which includes a cathode electrode 40, an anode electrode 45 spaced from the cathode electrode 40 and a gate electrode 43 disposed between the cathode and the anode electrodes 40, 45. A barrier 44 is disposed between the cathode electrode 40 and the anode electrode 45 thereby separating the two electrodes 40, 45. Generally, an insulating layer 42 is deposited on the cathode electrode 40 for supporting the gate electrode 43, i.e., the gate electrode 43 is formed on a top surface of the insulating layer 42. The insulating layer 42 defines a cylindrical hole (not labeled) therein for exposing the cathode electrode 40. An emissive material 41, such as carbon nanotube, is disposed in the cylindrical hole on the exposed cathode electrode 40. Furthermore, a phosphor material 46 is formed on a surface of the anode electrode 45 facing to the cathode electrode 40. In the illustrated structure, the phosphor material 46 represents a picture element for displaying. A picture element means a minimum unit of an image displayed by the FED (i.e., a pixel). In a typical color FED, the color picture is obtained by a display system using three optical primary colors, i.e., R (red), G (green), and B (blue).
In use, different voltages are applied to the cathode electrode 40, the anode electrode 45 and the gate electrode 43. Electrons 410 are emitted from the emissive material 41, and then travel through the cylindrical hole, finally reach to the anode electrode 45 and the phosphor material 46. Therefore, the phosphor material 46 is activated and a visible light is produced.
The above field emission device, however, has a low resolution. Because electrons extracted from the emissive material 41 are diverged away from a central axis of the phosphor material 46 when they travel to the anode electrode 45, thus, a spot that electrons bombard on the phosphor material 46 is enlarged. In addition, some of the diverged electrons are diverged at a large angle and bombard on a neighboring picture element (not shown), therefore an error display is occurred. Furthermore, a high voltage for extracting electrons from the emissive material is needed because of a large distance between the emissive material and the gate electrode.
Therefore, what is needed is a field emission device having a high resolution, lower voltage for emitting electrons, and a high emission efficiency.